Engineers make magnets behave like graphene

In a study published in Physical Review X, researchers from The Grainger College of Engineering at the University of Illinois Urbana Champaign demonstrated how specially designed two dimensional magnetic systems can follow the same equations that describe mobile electrons in graphene. This mathematical connection could influence the design of radiofrequency devices and also provide researchers with a powerful new way to analyze and engineer these materials.

"It's not at all obvious that there is an analogy between 2D electronics and 2D magnetic behaviors, and we're still amazed at how well this analogy works," said Bobby Kaman, the study's lead author. "2D electronics are very well studied thanks to the discovery of graphene, and now we've shown that a not-so-well-studied class of materials obeys the same fundamental physics."

Inspiration From Metamaterials and Graphene

The concept grew out of Kaman's work with metamaterials. These materials are engineered so that their larger scale structure produces behaviors that would not normally occur in the material's natural atomic arrangement.

Kaman, a materials science and engineering graduate student working in the research group of professor Axel Hoffmann, realized that both graphene electrons and microscopic magnetic excitations in so called magnonic materials behave like waves. This similarity raised an intriguing possibility. Perhaps a magnetic system could be designed so that it behaves mathematically like graphene.

"Graphene is unique because its conduction electrons organize into massless waves, so I was curious if altering the physical geometry of a magnonic material to look like graphene would make it act like graphene," Kaman said. "I thought it would maybe have a handful of similar properties to graphene, but the analogy was much deeper and richer than I expected."

Designing a Magnetic System That Mimics Graphene

To explore the idea, the researchers modeled a thin magnetic film containing tiny holes arranged in a hexagonal pattern. Within this structure, microscopic magnetic moments, known as "spins," interact and produce traveling disturbances called spin waves.

When the team calculated the energies of these spin waves, they discovered that their mathematical behavior closely matched that of electrons moving through graphene.

The system turned out to be even more complex than expected. Instead of a simple one to one analogy, the researchers identified nine distinct energy bands. These bands allow several types of behaviors to appear at the same time. Among them are massless spin waves similar to graphene's electron waves, as well as low dispersion bands associated with localized states and even topological effects that span multiple bands.

"What makes Bobby's work remarkable is that it makes a direct connection between an engineered spin system and a fundamental physics model," Hoffmann said. "Magnonic crystals are notorious for producing an overwhelming variety of structure- and geometry-dependent phenomena, most of which are cataloged without really being understood. The graphene analogy in this system provides a clear explanation for the observed behaviors."

Potential for Smaller Microwave Devices

Beyond its importance for basic physics, the research could have practical applications. The team believes the system may be useful in microwave technology used in wireless and cellular communication.

"One such device is a 'microwave circulator' that only allows microwave radio signals to propagate in one direction," Hoffmann explained. "They are usually bulky, but the magnonic system we studied could allow microwave devices to be miniaturized to the micrometer scale."

Hoffmann's research group has already filed a patent application covering their microwave device concepts.

Jinho Lim and Yingkai Liu also contributed to the research.

Support for the work was provided by the Illinois Materials Research Science and Engineering Center through the National Science Foundation.

Axel Hoffmann is an Illinois Grainger Engineering professor of materials science and engineering in the Department of Materials Science and Engineering. He is also affiliated with the Materials Research Laboratory and holds a Founder Professor appointment.